Gas assisted injection molding of plastic has long been known in the industry. Briefly stated, molten plastic is forced into an enclosed mold, and gas is injected into the mold within the plastic material. The gas raises the internal mold pressure, and creates an expanding gas pocket, which forces the cooling plastic to the extreme recesses of the mold, yielding a better fill out of the mold surface and reducing the sag of the plastic from the mold surface as the plastic shrinks during cooling, thus producing a better finished surface. The gas also creates an internal cavity within the molded part which reduces the weight of the part and reduces the amount of plastic required, thus reducing material cost.
There are a variety of methods and apparatuses for injecting gas into a mold, whether the gas is injected at a varying pressure and whether the gas is injected through the plastic injection nozzle or remotely. Remote gas injection locations include injecting directly into the mold cavity (in article) or into a channel leading into the mold (in-runner). Due to the higher gas pressures generally required for dual plastic/gas injection nozzles, and the associated expense for valving for the resin and gas flows, the injection of gas remotely is generally preferred.
Nozzles for use in-article or in-runner remote gas injection devices are subjected to packing by the molten plastic injected into the mold. Such gas injection nozzles are typically located near the plastic injection nozzle so that the gas injected can best assist the flow of the plastic material throughout its flow through the mold. This however typically subjects the gas injection nozzles to the flow of molten plastic at its most liquid state and highest pressure, which tends to clog or pack air injection nozzles. Further, gas injection nozzles are typically used as gas exhaust outlets, so that any molten material will tend to flow toward and into the outlet during the venting process. Cycle time of the molding process is critical to production cost, so venting before the interior of the part has completely cooled may be desirable, creating the potential condition for uncooled material flow toward the gas nozzle. To inhibit the flow of molten resin into the gas nozzle, two approaches have typically been used: a valved fluid nozzle (i.e. U.S. Pat. No. 5,232,711), or to use an injection pin with very small orifices which tend to resist the flow of the molten resin (i.e. U.S. Pat. No. 5,820,889). Another method employed to avoid the clogging of the gas supply passages with molten resin is to delay gas injection until the plastic injection is completed, as described in U.S. Pat. No. 5,295,800. However, this allows the plastic to cool somewhat, which reduces the flowability of the material, and reduces the efficacy and efficiency of the gas injection process.
The use of valved gas nozzles adds complexity and expense to the entire system. Injection molding is a relatively high production process, so such nozzles are subjected to repeated exposure to molten resin under pressure, which inevitably leads to the intrusion of resin into the inlet. Further, a valved nozzle typically includes a reciprocating motion opposing the intrusion of plastic or overcoming the gas injection pressure, a motion that requires a relatively large force which inevitably leads to wear and failure. Repairing or replacing such reciprocating nozzles or valves is time consuming and expensive in material cost and in down time. Examples of reciprocating nozzles or pins are shown in U.S. Pat. Nos. 4,740,150; 4,905,901; 5,151,278; 5,164,200; 5,198,238 and 5,464,342. Reciprocated nozzles or pins may limit material intrusion during material injection, but have little effect upon material intrusion during venting if venting is conducted through the nozzle.
The use of existing stationary pins is initially less expensive, but relies upon the relative inability of the molten resin to flow into small orifices due to its viscosity, thixotropy, and particle size. Examples include: U.S. Pat. No. 5,820,889, which describes a pin with multiple apertures between 0.0025 and 0.0006 inches in diameter arranged to form a right triangle pattern; U.S. Pat. No. 4,855,094, which discloses a disk shaped insert with an orifice sufficiently small to effectively prevent entry of molten plastic; and U.S. Pat. No. 5,284,429, which discloses the use of a porous plug.
Restricting the size of the inlet opening may limit the intrusion of the thermoplastic material, but will not completely eliminate material flow into the nozzle. Such material flow is typically limited to strings of material which will cool in the nozzle body, but which still must be removed. One approach described in U.S. Pat. No. 5,151,278 has been to combine a restricted orifice (a pin seated in a smooth nozzle barrel creating an annular gap of 0.0078xe2x80x3 for the flow of gas) and a reciprocating nozzle, such that the nozzle barrel is wiped clean when the nozzle reciprocates. Further steps to limit material intrusion are described in U.S. Pat. No. 5,164,200 to provide the pin seated within the smooth nozzle barrel with a variety of cross-sectioned configurations, including a hexagonal, oval, or threaded. These configurations are stated to create a turbulent air flow through the nozzle which throws the plastic outside of the path of gas flow, and thus restricting plastic intrusion.
Another similar approach of restricting material intrusion is described in U.S. Pat. No. 5,464,342, which provides a pin seated within a smooth barrel of the nozzle. The pin or multiple pins can be provided with enlarged diameter sections which would inhibit material intrusion into the barrel and, if it were desired to further restrict the flow of molten thermoplastic, a pin section having a threaded outer periphery. Additional pin/bore configurations are described in U.S. Pat. No. 4,905,901. The removable pin contains a threaded bore for gas flow into which a precision plug sized so that the exterior periphery of the plug contacts a crown portion of each thread to keep the plug in place. The flow is thus constrained to the helical threaded groove path and retards flow of thermoplastic material into the pin bore. The pin must be removed in order to remove the plugs or baffles or bluff bodies seated within the pin bore.
Such attempts to control the degree and effects of material intrusion illustrate the severity of the problem. While such efforts have achieved various degrees of success, they still require the disassembly of multiple parts for cleaning or replacement and require precisely machined or manufactured parts.
Another disadvantage of existing nozzle designs is the use of smooth bore conduit within the nozzle. If there is an over-packing situation, which can result if the molding control system fails to timely stop the material injection process, the plug that develops in the nozzle can be continuously pushed further into the smooth bore, and could eventually be pushed through the bore and into less accessible or more sensitive components. Further, replaceable pins for injection nozzles are known, but these smooth bore pins are difficult to clean, so they are typically discarded or xe2x80x9clostxe2x80x9d by the molding operator.
Another aspect of gas injection nozzle design is the adjustability of the gas flow in terms of both volume and direction. As discussed in several of the above-referenced patents, it is seen as an advantage to direct the nozzle opening in the direction of the material flow, both to direct the gas flow to assist the material flow and to shield the opening from the direct material flow and thus reduce the effects of packing. Effects of selecting the direction of the gas flow (whether injecting or exhausting) upon the flow of the plastic material is discussed in U.S. Pat. No. 5,482,609. Further, it is desirable to be able to adjust the width of the nozzle opening for various applications to create a wider or more narrow gas injection pattern. Similarly, it is preferable to be able to adjust the size of the nozzle opening to be able to adjust gas velocity at a given gas pressure introduced into the nozzle. No simple and satisfactory or cost effective manner of adjusting the nozzle opening between applications is currently available.
An example of the uncertainty in the industry regarding gas injection pressures is shown in U.S. Pat. Nos. 5,118,455 and 5,039,463, which refers to fluctuating the gas supply pressure. Such attempts fail to recognize the dynamic pressure conditions which occur at the gas injection nozzle opening. Gas exiting the nozzle opening will experience an immediate drop in pressure as the gas enters the mold cavity and dissipates. Continued flow of gas will raise the gas pressure in the cavity near the nozzle until an air pocket develops, the pressure within the air pocket exceeding the fluid pressure of the semi-fluid plastic and displacing the hardening plastic throughout the mold. The pressure within the air pocket will fluctuate depending on the pressure of the gas flowing into the nozzle, and the viscosity, injection pressure, flow rate and cooling rate of the plastic material as it fills out the mold. It is very difficult to calculate the effect of these variables, so it is preferable to have a gas injection nozzle opening that can be easily and inexpensively adjusted during the run off of a mold to maximize the molding process, or during a lengthy run of parts on a given mold as the variables change to xe2x80x9ctunexe2x80x9d the process back to peak performance.
Wherefore, the objects of the present invention are to provide inexpensive interchangeable pins for a gas injection nozzle for injection molding apparatus, such pins having inexpensive interchangeable outlet components allowing for ready adjustment of the nozzle opening in size, direction and shape or configuration. Further, it is an object to provide a baffled gas passageway to resist intrusion of molten material, and a removable threaded shank that will collect any intruding material or flash in a manner that can be easily cleaned.
The present invention includes a nozzle having an internally threaded bore communicating through a mold body with a source of a pressurized gas, and an externally threaded pin received within and engaging the threaded bore. The pin has a machined longitudinal surface creating an air passage through the bore when the pin is in place. At the outlet end of the nozzle the pin carries one or more washers having air passages machined therein. The internally threaded nozzle and externally threaded pin are inexpensively manufactured and easily replaceable. The outlet washers are very inexpensively manufactured and can be stacked, or otherwise arranged to provide a wide variety of outlet configurations.